Molecular mechanism of estrogen action to inhibit obesity

雌激素不仅对女性生殖器官有多种影响，而且对包括脂肪组织在内的非生殖器官也有影响。雌激素抑制去卵巢引起的啮齿动物肥胖。我们研究了雌激素依赖抑制肥胖的机制。在稳定表达雌激素受体(ER)的转基因3T3-L1脂肪细胞中，雌激素显著降低脂肪积累量和脂蛋白脂酶(LPL)mRNA以及甘油三酯的积累量。然后研究了雌激素对小鼠LPL基因的转录调控。将pLPL(1980)-CAT构建和ER表达载体一起导入分化的3T3-L1细胞。并测定CAT活性。Er，主要是配体依赖的，抑制了基础LPL启动子7倍的活性。我们利用一组pLPL-CAT报告基因的5‘-缺失突变体，在LPL启动子中寻找雌激素反应抑制元件。虽然没有经典的雌激素反应元件，但已证实位于(-1856/-1850)处的类似AP-1的TGGAATTC序列负责雌激素抑制LPL基因的转录。用TGAATTC序列探测的电泳迁移率改变分析表明形成了一种特定的DNA-核蛋白复合体。有趣的是，这种复合体不受添加的任何针对ER、c-jun、c-Fos、JunB或Jund的抗体的影响。这些结果可能表明存在一个独特的信号通路，它利用一个独特的雌激素反应元件，而不是经典的ERE和可能调节LPL基因转录的结合蛋白。

Estrogen exerts a variety of effects not only on female reproductive organs but also on nonreproductive organs, including adipose tissue. Estrogen inhibits obesity trggered by ovariectomy in rodents. We studied the mechanism underlying this estrogen-dependent inhibition of obesity. Estrogen markedly decreased the amounts of fat accumulation and lipoprotein lipase (LPL) mRNA as well as triglyceride accumulation in genetically manipulated 3T3-L1 adipocytes stably expressing the estrogen receptor (ER). Transcriptional regulation of the murine LPL gene by estrogen was then studied. A pLPL (1980)-CAT construct, along with an ER expression vector, was introduced into differentated 3T3-L1 cells. and CAT activities were determined. ER, mostly ligand-dependently, inhibited the basal LPL promoter activity by 7-fold. We searched the LPL promoter for an estrogen-responsive suppressive element by employing a set of 5'-deletion mutants of the pLPL-CAT reporter. Although there was no classical estrogen response element, it was demonstrated that an AP-1-like TGGAATTC sequence located at (-1856/-1850) was responsible for the suppression of the LPL gene transcription by estrogen. An electrophoretic mobility shift assay probed with the TGAATTC sequence demonstrated formation of a specific DNA-nuclear protein complex. Interestingly, this complex was not affected by the addition of any antibodies against ER, c-Jun, c-Fos, JunB, or JunD. These results may indicate the existence of a unique signaling pathway utilizing a unique estrogen response element other than classical ERE and binding proteins which possibly regulate transcription of the LPL gene.